The present invention relates to spin-type driver tools that are subject to torsional forces and more specifically to the shank portions to which turning forces are applied to such driver tools.
The most common of the driver tools of the type being discussed here include the well known hand-held spin-type driver tools such as screwdriver, nutdrivers, and allen hex drivers, to mention a few. Spin-type driver tools, however, also include tools adapted for use with power driving tools such as electric drills and impact mechanisms. All of these driver tools characteristically include a shank portion consisting of an elongated shaft, having at one end some type of driver means adapted to engage a complementary fastener in known manner. The other end of the shaft supports a handle grip or presents some type of engaging surface to which a turning force can be applied.
In the normal use of handled type driver tools, turning forces are applied to the handle grips which are coupled to the shafts so that they transmit torque to the driver means which in turn transfers these forces to a complementary fastener engaged by the head on the shaft. For the tool to be effective, the handle grip must not slip around the shaft when the turning forces are applied. Where the shaft about which a handle grip is fitted is tubular, there is nothing for the handle grip to engage to prevent it from slipping in response to the turning forces. Therefore such shafts are typically provided with some type of protrusion means to engage the surrounding handle material to prevent such slippage. Protrusion means positioned on the shaft are well known in the prior-art and many different arrangements of them are employed. For example, one prior-art arrangement utilizes a pair of oppositely disposed fin-members in recessed portions on the shaft. Another prior-art arrangement has the end of the shaft flattened to a screwdriver-type-head appearance. Yet another prior-art arrangement has the end of the shaft squared or otherwise configurated into a noncircular shape. Still another type uses small splines near the end of the shaft.
The handle grips used on driver tools are commonly made of plastic or wood because of the comfort, grippability and electrical insulating properties of these materials. From a manufacturing standpoint, handle grips made from these materials are best assembled with the driver shaft by the press-fit technique. According to this procedure, the handle grip is provided with an undersized axial bore in one end where the handle grip is forced onto the end of the driver shaft. The force with which this is done causes the handle material adjacent the axial bore to expand about the exterior surface of the shaft and then compress onto the shaft to form a snug fit therewith.
Driver shafts incorporating the prior-art protrusion arrangements mentioned above, however, have not proved effective for high torque applications when assembled with handle grips by the press-fit technique because they do not present an axially smooth surface about which the handle material can readily engage. This may be due to the fact that in many instances much of the surface area of the prior-art protrusions intended for torque transmission is crouched in axially recessed portions on the shaft or the protrusions are arranged too close together. This results in the formation of natural pockets adjacent the protrusions where the engaging handle material will not extend and hence will not contact the shaft.
The shortcomings of the prior-art protrusion arrangements become readily apparent in torsional tests conducted on plastic handled driver tools. Torque standards for hand tools of the type being discussed here, have been established by government agencies to provide some indication to measure quality and utility of hand tools. Among other things these standards specify torsional moment tests which tools must be able to withstand. The standards are subject to review and are amended from time to time. For example, the General Service Administration standards now require that handled driver tools for use by its agencies must be able to surpass torsional requirements of up to 250 in.-lbs. at an elevated temperature of 125.degree. Fahrenheit.
Handled driver tools equipped with the above mentioned prior-art protrusion arrangements are unable to meet the torque standards at elevated temperatures. The standard plastic handle grips used on handled driver tools experience some degree of softening at high temperatures. This loosens the fit between the handle grip and the shaft of prior-art devices, with the result that the driver fails.